[Refactor] Standardize Subscription Metadata Schema

**Title:** [Refactor] Define the Standardized SubNFT Metadata Schema (JSON)

### Description
We need a consistent JSON structure for the NFT metadata so that third-party marketplaces can display the expiration date correctly.

### Goal
Create a `schema/metadata.json` file defining fields like `expiration_timestamp`, `subscription_tier`, and `renewal_link`.
